Interpretive Summary: Toxoplasma gondii is a single-celled parasite of all warm-blooded hosts worldwide. It causes mental retardation and loss of vision in children, and abortion in livestock. Cats are the main reservoir of T. gondii because they are the only hosts that can excrete the resistant stage (oocyst) of the parasite in the feces. Humans become infected by eating undercooked meat from infected animals and food and water contaminated with oocysts.
Scientists at the Beltsville Agricultural Research Center and CDC, Atlanta, Georgia, report on the molecular epidemiology of Toxoplasma. These results will be of interest to veterinarians, public health workers, and biologists.

Technical Abstract:
Toxoplasma gondii is a protozoan parasite that infects nearly all mammal and bird species worldwide. Usually asymptomatic, toxoplasmosis can be severe and even fatal to many hosts including people. Elucidating the contribution of genetic variation among parasites to patterns of disease transmission and manifestations has been the goal of many studies. Focusing on the geographic component of this variation, we show that most genotypes are local specific, but some are found across continents and are closely related to each other, indicating a recent radiation of a pandemic genotype. Further, we show that the geographic structure of T. gondii is extraordinary in having one population that is found in all continents except South America whilst other populations are confined to South America, and yet a different population is found worldwide. Our evidence suggests that South American and Eurasian populations have evolved separately until recently when ships populated by rats, mice, and cats provided T. gondii with unprecedented migration opportunities probably during the Trans Atlantic slave trade. Our results explain several enigmatic features of the population structure of T. gondii and demonstrate how pervasive, prompt, and elusive the impact of human globalization on nature is.